Basting the meat while it cooks is important to add flavor. I like to mix up a simple cuban inspired mojo
of sorts made from olive oil, fresh garlic, fresh oregano, salt, pepper, orange juice and lemon juice. Baste the roasting animals every time you rotate the spit, or more.
There are many possibilities on marinades here and very few rules. In general, I have stayed away from typical BBQ sauces since they are thick and don't penetrate the meat very well. Additionally, you don't want to put anything on the outside of the pig that will burn over the many hours of cooking, so it's best to stay away from sticky, sweet glazes until the very end. If you do decide to glaze, keep in mind you'll only be treating the skin and not the meat.
Instead, I usually save the sweet sugar, tomato or honey based sauces until after the pig is carved, and pour them over the meat before serving, or let people serve themselves sauce as a side and just serve the meat un-sauced.